An antenna, also known as an aerial, is an electronic device that converts electric power into radio waves and vice versa. Antennas are used to transmit and/or receive radio frequency (RF) signals. An antenna element is an electrically conductive member of an antenna. Various arrangements of antenna elements are known, such as dipole, monopole, Yagi and helix, each arrangement having a characteristic radiation pattern, impedance, etc. For example, helical antennas are widely used for space communication, because helical antennas inherently transmit circularly polarized radio waves and can receive linearly polarized signals, regardless of the linear polarization orientation. This is important, at least in part because orientation of an antenna on a spacecraft changes as the spacecraft orbits or spins, thereby making it difficult or impossible to maintain linear polarization alignment between the spacecraft antenna and a ground-based antenna.
A directional antenna is an antenna that radiates greater power in one or more directions than in other directions. A directional antenna is correspondingly more sensitive to signals received from one or more directions than from other directions. A directional antenna may be physically aimed toward a receiving antenna, such as an antenna on a spacecraft, to concentrate transmitted power toward the receiving antenna, rather than directions that do not contribute to reception by the spacecraft or to reduce signal power toward an unintended receiver. Similarly, a directional antenna may be physically aimed to receive desired signals from a particular direction and reduce reception of unwanted interference by signals from other directions.
Mechanically reorienting an antenna imposes limitations on speed with which the antenna's orientation can be changed, accuracy of aiming the antenna, reliability of mechanical devices used to support and orient the antenna, etc. To overcome these and other limitations, some antenna arrays (also known as phased arrays) are steered electronically. Signals with particular phase relationships are fed to antennas of a phased array, such that constructive and destructive interference between radiated signals from the individual antennas of the array yield a radiation pattern that is reinforced in a desired direction and suppressed in undesired directions. The radiation pattern can be reshaped very quickly, enabling phased arrays to be used in radar systems to track multiple moving targets. However, phased arrays are much larger than a single antenna of such an array.
Phased arrays of helical antennas have been described in the prior art. For example, U.S. Pat. No. 6,243,052 by M. Larry Goldstein, et al. describes a phased array antenna having a spatially periodic array of helical antenna elements and RF feed circuitry.
A steerable beam helical antenna is described in U.S. Pat. No. 5,612,707 by Rodney G. Vaughan. The Vaughan device uses a furled dielectric sheet on which one or more conductors are fixed. By furling and unfurling the dielectric sheet, the antenna beam may be steered. However, furling and unfurling the dielectric sheet is a mechanical process, which suffers from the deficiencies mentioned above, with respect to mechanical antenna aiming systems.
Thus, while beam steering is important for many applications, it is often limited by space, overall size of the system and other factors.